Point Scatterer Model for RCS prediction using ISAR measurements

Alper Kaya, Mesut Kartal

Department of Electronics and Communication Engineering Istanbul Technical University

Istanbul, Turkey m_alper_kaya@yahoo.com, kartalme@itu.edu.tr

Abstract— A New Point Scatterer Prediction Model is presented in this paper to predict RCS of an aircraft model using Inverse Synthetic Aperture Radar (ISAR) images of the aircraft.. The ISAR images are provided by Defense Agency of Sweden and Saab inc. for each 15 degrees in horizontal plane. RCS is predicted for each aspect angle by taking superposition of the dominant point scatterers whose locations and strength are calculated from these ISAR images and simulation results are given with measurement results of the same aircraft model.

Keywords; RCS, Point Scatterers, ISAR, Aircraft

I. INTRODUCTION Point scatterer modelling is a technique of radar target

modeling based on approximation of electromagnetic scattering. The aim is to create equivalent point-scatterer model of the target, where each point scatterer is defined according to its position and Radar Cross Section (RCS) which is calculated from ISAR images. Then the contributions from all point scatterers are summed to simulate the target RCS. The focus of modeling is to define the amount and location of point scatterers for each aspect angle to estimate the RCS of target most accurately. Besides, it is considered that the less numbers of point scattereres used, the better the model efficiency is.

The basic principle of ISAR is that the object of interest is rotated to present a change in viewing angle to the fixed radar. The relative motion of the target results in Doppler frequency shifts being associated with different parts of the target. The technique utilizes a wideband, frequency-varying waveform for range resolution. Processing the individual wideband echo signals provides information on the relative range of individual scatterers on the target. By FFT processing of the frequency response, a high-resolution range profile of the target can be derived. This is the first process in generating an ISAR image. The resolution in cross range on the other hand, is closely dependent on the measurement of Doppler frequency shifts. An ISAR image is, mainly, a representation of the spatial distribution of the reflectivity of a target. The target’s reflectivity is usually mapped onto a down-range (range) versus cross-range plane, and is viewed as a radar image of the target. The image consists of small picture elements (pixels for example) that represent the target’s distributed reflectivity. The reflectivity is represented by different shades of color or grey-scale. The image may also be considered as a map of scattered

energy, as the RCS is directly related to the amount of scattered radiation from the illuminated target.

II. POINT SCATTERER MODEL The RCS measurements which are compared with the

simulation results of Aircraft Model Target were carried out in year 2000, by Saab Bofors Dynamics and the Federal Defense Agency of Sweden.[1]. The target was placed on top of a Styrofoam tripod, approximately 100 meters away from the antenna as in Fig. 1[1]. The radar antenna is heading towards to nose of model when azimuth angle is zero. The whole arrangement was placed on a large turntable tripod that rotated with constant angular velocity, while the radar was held stationary as a usual appliance for ISAR measurements. Pulsed Mk-V radar with 4.28 GHz bandwidth in the X-band was used to measure the radar cross section in the horizontal plane, as a function of azimuth angle.

Figure 1. Styrofoam Tripod

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In this paper, the locations and values scatterers to represent the target RCS are caareas which are %0.25th of image where the is 340X380 and the processed Images are 1flow of software is shown in Fig 2.

Figure 2. Flowchart

The input ISAR image for an aspect andefined by its Red, Green and Blue elements file block. Then Embedded Function 1 convertcorrespondent reflectivity values which are m(dB square meter). Maximum values of calculated at the next block. Then, the elemexcept the maximum are added to the maximumis assumed in the model that a peak shows sinand creates secondary and smaller hills in the shills are considered to the main hill and they Moreover, it occurs not only in the columscattering region around it although it is knownand wing behaves like dihedral corner reflectoback-scattering [2]. Thus, neighboring values are interpreted to be resulted from strong backand are added to each other, if they satisfy the cshould be 8 db difference in between whichdecided to prove that it’s not a stand alone bacThe values which have 8 db differences with are also summed to bigger one. Embedded funtake the opposite RCS values of points as the originally for image processing does not suppoAfter points having the value less than a levetotal RCS, which is saved in the workspac“RCS_30_5” for the example, is calculatesuperposition of all points while the coordinate

of strongest point alculated over unit ISAR images size

19X20 pixels. The

points are send to draw markpoint are also saved in the”image_30_5” in particular inmarkers block locates the coothe center of each unit image circles at those centers with adrawn at the upper left corner ooutput image is saved as Marke

ngle as in Fig.3 is by the Image from ts RGB elements to measured in dBsm each column are

ments of a column m of that column. It c form in a column same row and other are added onto in.

mn, a peak creates n that joint of body r and causes strong of point scatterers

k-scattering sources condition that there

h is experimentally k scattering source. neighboring points

nctions 3 and 4 only threshold designed

ort negative values. l are threshold, the

ce under the name ed by taking the

es of

Figure 3. S

The scatterer points are drwhose centers spot the center oin the title of each figure is tfrom 0 when the radar is headin

III. SIMULA The marked images and RC

given for the 2 and 5 point scatdB and 21.5 dB in Fig. 4 and Figreflectivity points for θ =30o. Thattachment is caused by dihedral

Figure 4. 2-Point

kers block. The values for each e workspace under the name n this example. Then ,the draw ordinates of points which are in area in the big image and draw a an additional reference circle of the output image. Finally, the edimage data in workspace.

Sample Image

rawn with circles on the image of the unit area. The angle stated the azimuth angle which starts ng towards the model nose.

ATION RESULTS S’s for the 300 azimuth angle are terers whose total RCS are - 21.1 g. 5 namely. There are two strong he one which is at the wing body l corner reflector.

ts Model for θ=300

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Figure 5. 5-Points Model for θ=30 Another one is the result of diffraction standing tail stabilizer. Diffraction effectproduces just a secondary effect on RCS. Thsaid as the secondary one that the edge of theimages with different numbers of points areshow the selection of strongest points in tcontribution when the number of poinSimulation results of other aspect angles forof points are given in Fig. 6

(a) θ=15 0

(c) θ=600

00 effect of vertical t of wing edges

he last point can be e nose. The output e given for 300 to the sense of RCS nts is decreased. r various numbers

(e) θ=900

(g) θ=1200

(b) θ=450

(d) θ=750

(i) θ=1500

Figure 6. Simulation Resu

After spotting strongest poincalculated for each aspect angleof points. Maximum number scatterers where bigger numberRCS for that aspect angle. Forconsidered as maximum numb6th points contributions to tnoteworthy. Maximum numbeeach aspect angle and takes vathe angle. 3 figures for each nFig. 9-11 to be compared wiresults in Fig. 8 which is prodthe aspect angles of RCS Measmultiples of 15 degrees..

(f) θ=1050

0

(h) θ=135

(j) θ=1650 ults for several aspect angles

nts on each image, RCSs are e for 1, 2 and maximum number indicates the number of points rs taken doesn’t change the total r example, 5 point scatterers are ber of points because addition of target RCS is not considered er is decided experimentally for alues from 5 to 10 depending on number of points are created in th Sampled RCS measurement duced by taking RCS values of surement Result in Fig. 7 [1] for

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Figure 7. RCS Measurement Results of air

Figure 8. Sampled RCS Measurement Results o

dBsm

10

scat

terer

0

-10

point

1 -20

for

RCS

-300 20 40 60 80 100 120 Aspect angle - degrees

Figure 9. RCS of 1 Point Scatter

dBsm

10 -

Scat

erer

s 0

-10

Poin

t

2 -20

for

RCS

-300

20 40 60 80 100 120 Aspect angle - degrees

Figure 10. RCS for 2- Point Scatte

Figure 11. Max RCS of Point Scatterer M The minimum error occurs in case of maxpoints are taken to consideration for each error chances between 7-10 dB and it stays s

rcraft target

of aircraft target

140 160 180

rer

140 160 180

erer

the angle range. This consisseveral reasons. A step in imimage, to smaller scale may beperformed by taking arithmetieach pixel in a unit area and cfrom that mean of color code arithmetic mean operation resuvalues which are in dB. Besidarea measurement is performeenvelope of RCS in Fig.11 isresult in Fig. 8

Some characteristic owith the results. One firstly ssame point locations represenfrom all angles. This fact suppon aspect angle. Second theimportance of geometrical shathat one corner point having ansame impact on RCS as the 8surfaces. Third, although the edcontribution to total RCS, the eof wings are much more clear tto strong effect of sharp edges

IV. CON This paper has described a wScatterer Model of an aircraftValidation will be completed ba real aircraft having variscattering behaviors. For futuvaluable for development of Fimage creation and processing

REFERENCES [1] Wessling A., Radar Target Mo

2002 [2] Sevgi, L., Complex Electro

Simulation Approaches, 2003, IE [3] N. A. Albayrak, Rcs Computat

Impedance Objects Modeled AsBilkent University

Model

ximum number of aspect angle. The teady for most of

sting error level is caused by mage processing; the resizing of e one of error source. Resizing is ic mean of RGB color code of calculates the RCS of the point representing that unit area. The

ult in greater change in the RCS des, the peak value of each unit ed outside. On the other hand, s quite similar to measurement

of RCS are also seen practically sees the impossibility to define nting the RCS characteristic of ports strong dependency of RCS e most noticeable fact is that ape on scattering. It can be seen n RCS around -20dBsm has the m2 perfectly conducting sphere dges of wings do not have a big effect of diffraction at the edges than the all surface of wings due on diffraction[3].

NCLUSION way of generating simple Point t for use in predicting its RCS. by a comparative analysis using ious different electromagnetic ure work, any contribution is

FFT error performance of ISAR resolution of the model.

odelling based on RCS Measurements,

magnetic Problems and Numerical EEE & John Wiley Press. tions with Po/Ptd for Conducting and s Large Flat Plates, Msc Thesis, 2005,

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